Mechanically-Controlled Chemical Zoning and its Preservation in High-Temperature Minerals from the Western Gneiss Region, Norway
Open access
Autor(in)
Datum
2019Typ
- Doctoral Thesis
ETH Bibliographie
yes
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Abstract
Chemical zoning in metamorphic minerals reflects variations in pressure and temperature along the path which the rock experienced. In many mineral phases, chemical diffusion is sufficiently fast to homogenize the chemical heterogeneities on geological time scales at temperatures exceeding 700°C. However, when chemical zonation in minerals that experienced high temperature conditions for millions of years is preserved, the current knowledge of chemical diffusion rates in minerals may not be sufficient to explain the preservation. Because in such cases, classical diffusion predicts complete chemical equilibration. Therefore, in high-temperature metamorphic rocks, the preservation of chemical zoning still remains enigmatic. Interestingly, the development and preservation of compositional zoning in minerals can be strongly influenced by mechanically-maintained pressure variations. To investigate these effects on the chemical zoning, samples are targetly chosen from a region that witnessed a long lasting high temperature metamorphism - the Western Gneiss Region (WGR). In the northernmost part of the WGR, temperature and pressure conditions were the highest within the WGR. This region consists of numerous mafic enclaves that have been well studied. However, the pressure-temperature evolution as well as the timing of the main metamorphic events of the host gneisses remains unknown. This area offers an excellent opportunity to study those host gneisses and explore their relationship to the ultra-high pressure (UHP) conditions attained in the mafic lenses. Phase equilibria modelling and Lu-Hf geochronology was applied with the focus on quartzofeldspathic samples. Interestingly, garnets from these samples have pronounced compositional zoning. Lu-Hf dating of compositional zones garnets reveal that investigated rock specimens experienced Caledonian ultra-high pressure (U)HP metamorphism with peak metamorphic conditions of >800 °C and 3.2 GPa, and a post (U)HP decompression. These samples are further investigated for mechanisms that might be responsible for the preservation of chemical zoning in high-temperature minerals and a comparison of classical diffusion modelling to approaches based on equilibrium under pressure gradient in natural garnets is provided. The ability to preserve the chemical zoning despite of the high temperatures is investigated. Results from the diffusion modelling show too short duration of the high-temperature event which is inconsistent with the existing regional geology data. On the contrary, the new equilibrium approach that predicts compositional zoning as a result of spatially varying pressure has been applied. All samples were systematically tested whether the observed chemical zoning can be fit by the new equilibrium approach. The results show a good fit with naturally observed chemical zoning in multi-component systems. This points to an explanation for chemical zoning in multicomponent systems, such as garnet, being preserved mechanically under high-temperature conditions, i.e. by local pressure variations.
The enigmatic preservation of an oscillatory zoned plagioclase inclusion in garnet lead to a detailed microstructural characterisation of the sharp zoned inclusion. To better understand how such a microstructure with sharp chemical zoning can be preserved on the million years’ time scale at high temperatures, two different diffusion modelling approaches were applied. The classical Fickian diffusion model predicts be homogenized within thousands of years. In contrast, when chemical diffusion is coupled with mechanical deformation, a significant intragranular pressure gradient is developed at the very early stage of the chemical re-equilibration. Due to slow viscous relaxation, further chemical homogenization is inhibited, which may have contributed to the preservation of the oscillatory zoning. In this thesis, I investigated mechanisms that might be responsible for the preservation of chemical zoning in high-temperature minerals. The presented case studies show more evidence for the preservation of chemical zoning by mechanical effects. The results presented here may serve as inspiration to re-investigate the preservation of such a zoning in other high-temperature regions worldwide. Mehr anzeigen
Persistenter Link
https://doi.org/10.3929/ethz-b-000353450Publikationsstatus
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Beteiligte
Referent: Tajčmanová, Lucie
Referent: Vrijmoed, Johannes C.
Referent: Moulas, Evangelos
Referent: Baumgartner, Lukas
Referent: Eglington, Tim
Verlag
ETH ZurichOrganisationseinheit
09503 - Tajcmanová, Lucie (ehemalig) / Tajcmanová, Lucie (former)
ETH Bibliographie
yes
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